The present invention relates to a magnetic recording method for a digital signal, and more particularly to the improvement of a magnetic head driving current waveform.
In a magnetic recording method for a digital signal which utilizes a magnetic medium such as magnetic tape and magnetic disk, recording is effected in such a way that a magnetic head is driven by current corresponding to recording information of "1" or "0" given in a time sequence, so as to form a magentic pattern on the magnetic medium. The correspondence between the information of the digital signal and the driving or recording current is called the recording system, and there are recording systems such as the NRZ, NRZI, MFM and, M.sup.2 system.
In general, in prior-art methods of recording digital signals, a rectangular wave is used as the magnetic head driving current for any of these recording systems. By way of example, FIGS. 1(a), 1(b) and 1(c) refer to the NRZI recording system and show recording information of (010), a driving current waveform and a playback signal waveform at the time when a magnetic pattern is read out, respectively. It is well known that the playback signal becomes a mountain-like pulse waveform with respect to one reversal of the driving current.
In a magnetic disk device, magnetic tape device, etc. for recording and playing back digital signals, in order to restore the playback signal to a signal corresponding to the original recording information, there is used a playback method which compares the level of the playback signal with reference to an amplitude detection level VTH indicated at numeral 6 in FIG. 1(c). This is called the amplitude detection. The time interval of a bit as indicated by letter T in FIG. 1(a) is termed the bit period, and the level comparisons are made at the centers of the bit periods T as indicated at numerals 1, 2 and 3 in FIG. 1(b).
In this regard, when the line recording density of the recording medium becomes high, usually the playback pulse waveform corresponding to one reversal of the recording current becomes a waveform spreading wider than the bit period, and the amplitude of the playback signal does not become zero at the centers of the adjacent information bits of "0", as shown at numerals 4 and 5 in FIG. 1(c). This phenomenon is called the intersymbol interference, and when it has increased, a large number of symbol errors develop in the amplitude detection.
In the magnetic recording device etc. for recording and playing back the digital signals, therefore, a circuit for compensating the waveform after the playback is provided so as to equalize the playback waveform. FIG. 2(c) illustrates the waveform compensation based on the equalization. FIGS. 2(a) and 2(b) show a recording information signal and a playback signal, respectively. One playback pulse waveform is equalized so that the amplitudes of the points 7 and 8 of the adjacent bit periods may become zero as indicated at numerals 9 and 10. Such equalized waveform is called the Nyquist waveform. When the operation of the equalization is discussed from consideration of the frequency characteristic, the equalization serves to make the frequency characteristic of the playback pulse waveform the Nyquist frequency characteristic. This will be explained with reference to FIGS. 3(a) and 3(b). The frequency characteristic of one spreading playback pulse is such that, as indicated at numeral 11 in FIG. 3(a), a higher frequency component is smaller in amplitude than a lower frequency component. This frequency characteristic is compensated so as to become a frequency characteristic 12 in FIG. 3(b) as indicated by: ##EQU1## phase delay magnitude being constant, EQU when.vertline.f.vertline..ltoreq.(1+b)f.sub.o /2
This frequency characteristic is called the Nyquist frequency characteristic, and a waveform on a time axis corresponding thereto is the Nyquist waveform. Here, f denotes the frequency, A the amplitude, b (0.ltoreq.b.ltoreq.1) the roll-off, and f.sub.o /2 the Nyquist frequency.
Such an equalizing method has been performed in the so-called linear transmission channels of a telephone channel etc., and the linear intersymbol interference (4 and 5 in FIG. 1(c)) thus far described can be perfectly eliminated by carrying out the equalization.
In a transmission channel for magnetic recording and playback, however, nonlinear intersymbol interference due to the interaction between bits occurs besides the linear intersymbol interference which can be compensated by the playback equalizer circuit. This will be explained with reference to FIGS. 4(a)-4(e).
FIGS. 4(a) to 4(e) show recording information of "0110", a recording current and equalized playback signals by taking the NRZI recording system as an example. An equalized playback signal 1 is obtained in correspondence with only the reversal 13 of the recording current, while an equalized playback signal 2 is obtained in correspondence with only the reversal 14 of the recording current. A waveform obtained by superposing the playback signal 1 and the playback signal 2 ought to become as shown at numeral 15 in FIG. 4(e). In actuality, however, when the pattern is recorded and then played back and equalized, a distorted waveform deviating from the superposed waveform develops as shown at numeral 16 in FIG. 4(e). Such distortion comes from nonlinearity peculiar to the magnetic recording, and is ascribable to the fact that the interaction between bits takes place due to recording demagnetization in the recording process or a demagnetizing field appearing from the recording medium. In addition, the nonlinear distortion differs in size in dependence on recording patterns and has the property of becoming greater with the line recording density of the recording medium.
In the presence of such nonlinear distortion, the amplitude of the playback pulse signal becomes different every recording pattern which is the combination of "1" and "0", and symbol errors develop frequently in the amplitude detection. Accordingly, making the amplitude of the playback pulse signal constant irrespective of the recording patterns is very important for reducing the symbol errors at the playback.
In order to evaluate the nonlinear distortion, an eye pattern is used. It is displayed by superposing equalized playback signals arrayed in a time sequence, at respective bit periods. It is exemplified in FIGS. 5(a)-5(e).
FIGS. 5(a) to 5(e) show a recording information, a recording current, equalized playback signals and eye patterns by taking the NRZI recording system as an example. The reversals 17 and 18 of the recording current in FIG. 5(b) are the same reversals of from a lower level to a higher level. Since, however, the respective adjacent recording current components differ as being "0" and "1", and "1" and "0", equalized playback signals come to have unequal amplitudes as respectively shown at numerals 19 and 20 in FIG. 5(c). Therefore, when the playback signals are displayed as the eye pattern, the eye portion of a hatched part 21 becomes small as shown by an eye pattern 1 in FIG. 5(d). In the amplitude detection, the level comparison is performed by setting the detection level VTH at the center of the eye. When this eye is small, a margin for the signal-to-noise ratio lessens, and symbol errors increase.
An eye pattern 2 in FIG. 5(e) is an equalized eye pattern synthesized by superposing one playback waveform, and shows a state free from the nonlinear distortion. When an eye is large as shown at numeral 22 in the figure, the signal-to-noise ratio has a wide margin, and there are few symbol errors in the amplitude detection.
Accordingly, to bring the eye pattern 1 having the nonlinear distortion close to the eye pattern 2 is an indispensable condition in points of enhancing the reliability of playback signals and rendering the density high.
As a measure for removing the distortion of the playback signal in such magnetic recording and playback, there have heretofore been proposed methods in which additional current reversal points are provided as shown in FIGS. 6(a) and 6(b) and FIGS. 7(a) to 7(c) (Japanese Patent Application Publication No. 55-40921). Referring to FIGS. 7(a) to 7(c), as regards recording information in FIG. 7(a), playback waveforms which are obtained in correspondence with the reversals 23, 24 and 25 of a recording current in FIG. 7(b) are as shown at 23', 24' and 25' in FIG. 7(c), respectively, and a playback pulse signal is narrowed on the basis of a waveform (23'+24'+25') obtained as the sum of the aforementioned playback waveforms.
By narrowing the playback pulse signal, these methods exhibit some effects for reducing the distortions of the playback waveforms attributed to the linear intersymbol interference shown at 4 and 5 in FIG. 1(c) and the nonlinear intersymbol interference shown at 16 in FIG. 4(e). These methods, however, relieve the interaction between bits by taking note of only the reversal of one recording current and provide for the spreading of one bit thereof and do not take into account the differences of the nonlinear distortions dependent upon the recording patterns, and the differences being the peculiar property of the magnetic recording and playback. Moreover, on account of the property that the transmission channel of the magnetic recording and playback cuts off shorter wavelength components as the line recording density of the recording medium becomes higher, there is the disadvantage that the effect owing to the additional current reversal points 24 and 25 in FIG. 7(b) lowers.